The need to detect contraband, such as drugs and explosives, is well appreciated. Efforts to detect contraband from being smuggled through various ports of entry, such as airports, border crossings and boat docks, has been a focus of attention. Various non-intrusive scanning techniques have been developed in the art which are more accurate than contemporary X-ray scanning techniques. It is known that nitrogen is a common element found in many illicit drugs and explosives. As such, nitrogen detection systems have been developed to detect nitrogen containing contraband.
A type of nitrogen detection system utilizes gamma rays. Generally, such a system uses a beam of energetic protons which are focused upon a target. The incident proton beam excites the target material according to well known principles, thereby causing it to produce gamma rays.
In this regard, it is known that when about 1.75 MeV protons impinge on a suitable target, e.g., a material coated with .sup.13 C, they have a high probability of producing 9.17 MeV gamma rays by the reaction .sup.13 C(p, y).sup.14 N. These gamma rays are emitted from the target nonuniformly at all angles. Those gamma rays emitted at about 80.66.degree., with respect to the direction of the proton beam, have a large probability of being resonantly absorbed by .sup.14 N contained in an object of interest. Detection of such absorption phenomenon is used to analyze the amount of nitrogen in an object of interest in order to detect nitrogen containing contraband.
As depicted in FIG. 1, a typical configuration of a prior art proton beam target consists of a thin film of .sup.13 C which is used to produce gamma rays. This gamma reaction layer is formed onto a proton stopping layer via an electron beam (or e-beam) evaporation process. The stopping layer is used to prevent undesirable transmission of energetic protons after they have traversed through the .sup.13 C gamma reaction layer. Because the incident proton beam results in the generation of substantial heat energy within the target, the stopping layer is attached to a cooling support for transferring heat energy away from the gamma reaction and stopping layers. The cooling support is typically formed of Copper or Copper alloys or Beryllium.
The stopping layer is formed of a suitable high atomic number (z) material. The high z material is required to effectively prevent the transmission of energetic protons. In this regard, the high z stopping layer is required to be of a minimal thickness necessary to fully stop the proton beam. The stopping layer, however, is also desired to be less than a thickness which substantially attenuates the gamma signal generated by the .sup.13 C gamma reaction layer. The stopping layer must additionally be formed of a material which will not react with the high energy proton beam to produce additional gamma signals which will interfere with the desired .sup.13 C resonant gamma emission. In addition, the stopping layer must survive the operating temperatures of the target. Thus, for example, the prior art has been to use a stopping layer formed of gold (Au) which is electro-plated onto a cooling support to a thickness of roughly 20 microns.
The desire to decrease the inspection or scanning time has driven the need to increase the operating current of the proton beam. Previously, due to the limited proton beam operating currents, prior art configurations have typically utilized proton beams operating at currents on the order of 10 micro-amperes. Proton beams have now been developed which are capable of operating at currents of 10 milliamperes (mA), three orders of magnitude greater than prior art devices. Prolonged exposure of the above described prior art targets to such high current protons, however, results in blistering and delamination of the prior art gold stopping layer contained therein, as well as, blistering and delamination of the outer .sup.13 C layer. Blistering describes the phenomenon wherein the incident beam of protons result in implantation of hydrogen atoms into the gold stopping layer. The implanted hydrogen tends to coalesce to form bubbles and causes the stopping layer to blister and delaminate the .sup.13 C coating/stopping layer interface from the stopping layer/cooling layer interface. As a result, the generated gamma rays are of an undesirable quality and nature and of a greatly reduced quantity. Thus, while prior art targets have been effective while used with proton beam currents of 10 micro amperes, such targets are inadequate when used with relatively high current proton beams.
It is therefore evident that there exists a need in the art for a proton beam target which is able to withstand exposure to the bombardment of high current protons while producing the desired gamma emissions.